Collaborations, Education and Outreach

1. Stabilizing Unstable Transition Metal Dopants in a Magnetic SemiconductorFrank Tsui, University of North Carolina,DMR 0441218 Our study on epitaxial films of Co and Mn co-doped Ge magnetic semiconductor (DMS) provides the first direct evidence of enhanced dopant stability, and atomistic mechanism for significantly enhancing doping level in a DMS. Systematic investigation using combinatorial approaches reveals that co-doping with Co can dramatically reduce phase separation and diffusion of Mn within Ge lattice, and that substitutional Co with its strong tendency to dimerize with interstitial Mn plays a critical role in controlling the energetics and kinetics of the system. The work demonstrates feasibility for tailored DMS materials and provides the impetus for developing atomistic approaches to elucidate group IV-based DMS. Phase diagram and strain states of Co and Mnco-doped Ge (001) epitaxial thin film.Regions I & II exhibit coherent epitaxy, where the film strains with respect to Ge obey Vegard’s law, and are free of detectible disorders. Region III contains Mn surface aggregation (blue contours), which is suppressed by Co co-doping. Region IV contains stacking faults (green contours and gray area) without detectible secondary phases.

2. Stabilizing Unstable Transition Metal Dopants in a Magnetic SemiconductorFrank Tsui, University of North Carolina,DMR 0441218 • Collaborations, Education and Outreach • L. He completed his PhD thesis, and B. Collins, W. Rice, and C. Malmberg received MS • Another graduate student M. Wolboldt and REU students, A. Mellnick and J. Haller, (5 total during the last 2 years) contributed • 2 high school students, Colin Smith (Emerald Isle NC) and Sophie Liu (Wilmington NC) participated (in collaboration with NC School of Science & Math and NC GlaxoSmithKline Foundation) • Students ran experiments at Argonne National Laboratory • REU student A. Mellnick received an undergraduate research fellowship with M. Wolboldt as the graduate mentor (2007) • B. Collins received the Best Student Poster award at the 24th North America MBE Conf. • REU student A. Fuller received highest honor for his honor’s thesis and won the departmental most outstanding senior award (2006) Layer-by-layer epitaxial growth of superlattices containing Ge DMS and Ge.RHEED (reflection high energy electron diffraction) patterns (a-c) and oscillations in intensity (close symbols) and width (open symbols) during Co and Mn co-doped Ge (blue) and Ge (red) growth, demonstrating the ability for controlled synthesis on atomic scale. XRD (x-ray diffraction) pattern (d) exhibits superlattice side bands, indicating high crystalline quality with sharp interfaces between DMS and Ge.